In medium-sized or large-sized liquid crystal display devices, thin-film transistors (TFTs: Thin Film Transistors) formed of non-crystalline silicon (hereinafter referred to as amorphous silicon) on a glass substrate are used for pixel switches.
In a related-art liquid crystal display device, the TFTs are used only for the pixel switches, and a semiconductor chip is used for each of a scanning line driving circuit for driving a scanning line and a signal line driving circuit for driving a signal line. This semiconductor chip is mounted on a flexible printed circuits (FPC: Flexible Printed Circuits) board or a flexible cable for transmitting a signal to the display device from an externally mounted printed circuit board or from a printed circuit board.
However, in recent years, reduction of the costs of a semiconductor IC and the printed circuit board and the cost of mounting the semiconductor IC and the printed circuit board is considered in order to reduce the manufacturing cost of the liquid crystal display device.
There is a method in which using TFTs formed of amorphous silicon, the scanning line driving circuit is included on the glass substrate of the display device, for example. According to this method, during the course of forming the TFTs on the glass substrate, the scanning line driving circuit as well as the pixel switches can be manufactured. Accordingly, the need for the semiconductor chip for the scanning line driving circuit and the need for the flexible cable for transmitting to the display device a pulse for driving a scanning line are eliminated. The need for a device and a step for mounting these semiconductor chip and flexible cable is also eliminated. As described above, by including the scanning line driving circuit on the glass substrate using the TFTs, the manufacturing cost of the display device itself is reduced.
In general, the scanning line driving circuit formed of the TFTs is formed of transistors of a single conductivity type (or a single channel). It is because, with this arrangement, the number of steps associated with channel characteristics can be reduced by approximately half. Reduction of the cost can be thereby achieved. As an example, Patent Document 1 describes a single channel driving circuit using amorphous silicon TFTs.
Further, each of Patent Documents 2 to 4 describes that a function of bringing a scanning line of a liquid crystal display device into a high-impedance state is demanded for a scanning line driving circuit.
Patent Document 2 describes a display device including a function of switching the scanning direction of a gate line. This display device includes two scanning line driving circuits each formed of amorphous silicon TFTs. This display device uses a method in which one scanning line driving circuit is brought into a high-impedance state and the other scanning line driving circuit is brought into an active state.
Patent Document 3 describes a display device in which, when a defective portion such as a line disconnection is included in a gate line or when one of stages of a gate driving unit does not function, a normal function of the display device can be swiftly recovered just by a simple repair. This display device includes a main gate driving unit and a sub gate driving unit. Then, switching units are provided. Usually, the sub gate driving portion is brought into a high-impedance state. When a defect is found, one of the switching units connected to the portion of the defect is turned on.
Patent Document 4 describes a display device of a touch sensor incorporated type suitable for reduction of weight, size, and thickness. This display device has a feature in which a scanning line is brought into a high-impedance state.